Quantum cascade (QC) lasers are known. See, for instance, U.S. Pat. Nos. 5,457,709 and 5,509,025. See also J. Faist et al, Applied Physics Letters, Vol. 68, p. 3680 (1996), all incorporated herein by reference.
Briefly, a prior art QC laser comprises a multiplicity of identical "repeat units", each repeat unit comprising an active region and an injector/relaxation region. With each active region is associated an upper and at least one lower energy level. Under an applied field, charge carriers (typically electrons) migrate from a lower energy level of a given active region through an injector/relaxation region to the upper energy level of the adjacent downstream active region, followed by a radiative transition from the upper to a lower level of the active region, then proceeding through an injector/relaxation region into the next active region, and so on. Thus, each charge carrier that is introduced into the relevant portion of the QC laser ideally undergoes N transitions (N being the number of repeat units, about 25, for instance), each such transition resulting in emission of a photon of wavelength .lambda., typically in the midinfrared (exemplarily 3-13 .mu.m).
A variety of QC designs have been disclosed. For instance, the above referenced '025 patent discloses a design with "vertical" transition, i.e., a transition from an upper to a lower energy level of a given quantum well (QW), and U.S. patent application Ser. No. 08/744,292, filed Nov. 6, 1996 by Capasso et al. discloses a design with superlattice (SL) active region.
A variety of injector/relaxation (I/R) region designs have also been disclosed. See, for instance, U.S. Pat. No. 5,727,010, which discloses a chirped superlattice (SL) I/R region.
Considerable performance improvement has been achieved since the invention of the QC laser in about 1994. For instance, J. Faist et al., IEEE Journal of Quantum Electronics, Vol. 34, p. 336 (1998), disclose a QC laser emitting at .lambda..about.5 .mu.m with continuous wave output power of 200 mW/facet at 80K. The laser had a modulation-doped "funnel" injector and a three-well vertical transition active region.
In view of the considerable economic and technological potential of QC lasers (e.g., for a variety of sensing and monitoring applications), it would be desirable to find still further ways to improve the performance of QC lasers. This application discloses such a further way.